Southeast Idaho Phosphorus Slag Program

Slag has historically been used extensively in Southeast Idaho for construction purposes as aggregate in concrete and asphalt, roadbed fill, backfill, and railroad ballast. In the 1950s, '60s, and early '70s, it was also used in the concrete poured for some basements and building foundations.

When elemental phosphorus is produced, it is removed from a mixture of phosphate ore, silica, and coke. The largest remaining by-product is a lava-like rock known as "slag." Primarily a compound of calcium and silica, slag also contains small quantities of uranium and radium. These two elements are naturally present in the phosphate ore. Their presence in the slag causes it to emit very low levels of gamma radiation - a type of radiation similar to medical x-rays.